/Engine/Source/Runtime/Core/Public/Containers/Set.h
Add all items from another set to our set (union without creating a new set) Compatible element type version.
template<typename OtherKeyFuncs, typename AliasElementType, typename std::enable_if_t< [TIsContainerElementTypeCopyable](API\Runtime\Core\Containers\TIsContainerElementTypeCopyable)< AliasElementType >::Value > *> void Append ( [TSet](API\Runtime\Core\Containers\TSet)< typename [TContainerElementTypeCompatibility](API\Runtime\Core\Containers\TContainerElementTypeCompatibili-)< [ElementType](API\Runtime\Core\Containers\TSet\ElementType) >::CopyFromOtherType, OtherKeyFuncs, Allocator > && OtherSet )